In the art of electron photography, image forming method that uses laser beam is used extensively as the image forming method capable of achieving image recording of high-definition image quality. In electron photography, a latent image is formed by causing a laser beam to scan over a photosensitive drum in an axial direction thereof (main scanning of the laser beam) by using a polygonal mirror, while rotating the photosensitive drum at the same time about a rotational direction thereof (sub-scanning of the laser beam).
In such a technological field of electron photography, there is a continuous demand of higher resolution and higher output speed. In the case resolution of image has increased by two times, the duration of two times as large as the duration needed for image forming process of conventional resolution becomes necessary for each of the main scanning process and the sub-scanning process, and thus, the duration of four times as large as the duration in the case of conventional image forming process becomes necessary with such high-resolution image forming process. Thus, for realizing high resolution image forming process, there is a need of achieving high-speed output of images at the same time.
For achieving such high-speed image formation, it is conceivable to use high laser beam output, multiple laser beam construction, high-sensitivity photosensitive bodies, and the like. Thus, it is generally practiced in the art of high-speed image forming apparatuses to use a writing optical source that produces multiple laser beams. With this approach, the area where the latent images are formed becomes n times as large as the conventional case of using a single laser beam when n laser beams are used simultaneously. Associated with this, it becomes possible to reduce the time needed for image formation to 1/n.
For example, there is a proposal of a multiple-beam laser diode (Patent References 1 and 2) that includes plural optical sources on a single chip. However, these conventional constructions use edge-emission laser diodes disposed to form a one-dimensional array, and because of this, there is a drawback of large electric power consumption, which in turn necessitates the use of a cooling system. From practical viewpoint of cost, the system of four beams or eight beams is thought as the limit of such an approach. Further, when the number of the laser beams is increased, there tends to arise a large deviation of the laser beam from the optical axis of the optical elements constituting the optical system, leading to degradation of optical properties.
On the other hand, a surface-emission laser diode is a semiconductor laser device that emits a light perpendicularly to the substrate and has an advantageous feature of easy integration to form a two-dimensional array. Further, as compared with the laser diodes of edge-emission type, a surface-emission laser diode has an advantageous feature of small electric power consumption, which is ten times as small as compared with the edge-emission laser diodes. Thus, use of surface-emission laser diode is thought to be advantageous when integrating a large number of optical sources to form a two-dimensional array.
For example, there is a known surface-emission laser array designed for a writing optical system that includes 32 surface-emission laser diode elements arranged in eight rows and four columns and uses a polygonal mirror for causing the scanning of the laser beams (Non-Patent Reference 1).
With this surface-emission laser array, eight of the surface-emission laser diode elements are aligned in the sub-scanning direction and fourth surface-emission laser diode elements are aligned in the main-scanning direction. Thus, designating the interval between each neighboring pair of the eight surface-emission laser diodes aligned in the sub-scanning direction (direction of drum rotation) as “d” and designating the interval between each neighboring pair of the four surface-emission laser diodes aligned in the main scanning direction (longitudinal direction of the drum) as “x”, the 32 surface-emission laser diode elements are disposed such that the interval between four straight lines drawn perpendicularly to a line extending in the sub-scanning direction from 4 respective centers of the four surface-emission laser diode elements aligned in the main scanning direction becomes equal and takes a value of d/4, and such that d is smaller than x (d<x).
With this, high-density writing with the density of 2400 dpi (dot/inch) is realized. Further, in the case the main scanning by polygonal mirror is not used and the optical sources are disposed in one-to-one correspondence as in the case of LED (light emitting diode) printer described in Patent Reference 3, the main scanning direction and the sub-scanning direction are interchanged.
Further, there is a known surface-emission laser array designed for a writing optical system that includes 36 surface-emission laser diode elements arranged in six rows and six columns and uses a polygonal mirror for causing the scanning of the laser beams (Patent References 4 and 5).
With this surface-emission laser array, six of the surface-emission laser diode elements are aligned in the sub-scanning direction and 6 of the surface-emission laser diode elements are aligned in the main-scanning direction. Thus, designating the interval between each neighboring pair of the six surface-emission laser diodes aligned in the sub-scanning direction (direction of drum rotation) as “d” and designating the interval between each neighboring pair of the six surface-emission laser diodes aligned in the main scanning direction (longitudinal direction of the drum) as “x”, the 36 surface-emission laser diode elements are disposed such that the interval between six straight lines drawn perpendicularly to a line extending in the sub-scanning direction from six respective centers of the six surface-emission laser diode elements aligned in the main scanning direction becomes equal with each other and takes a value of d/6.
Thus, in the case of concentrating the 36 laser beams emitted from the 36 surface-emission laser diode elements thus disposed by using a single collimating lens, it is preferable that that all the laser beams are gathered in the vicinity of the optical axis of the collimating lens for avoiding aberration of the lens. Thus, it is preferable that the surface-emission laser diode elements constituting the surface-emission laser array of the form of two-dimensional array are disposed with high integration density as high as possible. In view of the foregoing demand, there is a proposal of increasing the density of the plural surface-emission laser diode elements (Patent Reference 6). In Patent Reference 6, the plural surface-emission laser diode elements are disposed with a constant interval with each other.